1. Field of the Invention
This invention relates generally to tension leg platforms for deep-sea hydrocarbon production and specifically to mooring tendons therefor.
2. Description of the Prior Art
Tension leg platforms (TLPs) are increasingly used for exploitation of deep sea hydrocarbon reserves. A TLP is a semi-submersible floating platform anchored to a foundation on the sea bed by mooring elements, often called tension legs, tethers, or tendons. The tendons are maintained in tension at all times by ensuring net positive TLP buoyancy under all environmental conditions. The tendons stiffly restrain the TLP against vertical offset, essentially preventing heave, pitch and roll, yet they compliantly restrain the TLP against lateral offset, allowing limited surge, sway and yaw.
As shown in FIG. 1, the TLP has a submerged hull (14). The hull has a keel (24) and a top (48). The hull (14) has one or more vertical columns (20) extending upwards thereon that penetrate the surface of the water when the TLP is at installed draft. The columns generally support an integrated platform superstructure (not illustrated), which consists of one or more decks for drilling, production and processing equipment, support structures, and human use.
Each hull (14) is designed to mate with a number of tendons (12) at tendon porches located near the keel (24). The tendon porches contain connection sleeves (22) to receive and clamp tendons at the length adjustment joint (LAJ) (27), which are located at upper ends of the tendons (12). The connection sleeves (22) are often ring-shaped, requiring vertical entry of the tendons, or are slotted, allowing side entry of the tendons. The tendons (12) are usually made of hollow steel pipes. Steel pipe tendons are frequently watertight and internally sealed from the sea environment, filled with air at atmospheric pressure at sea level to reduce their weight in water and the resultant loading on the TLP. The tendons (12) terminate at their lower ends with bottom latch assemblies (50). The bottom latch assemblies form “stab” connectors that are received and locked into pilings (52) in a seabed foundation structure (54). The bottom latch assemblies (50) are usually designed to allow some tendon pivoting with respect to the foundation structure to accommodate limited lateral motion of the TLP due to wind, waves and currents.
The tendons (12) often accommodate tendon support buoys (TSBs) (30), which are temporarily secured to upper portions of the tendons to provide positive buoyancy and maintain the tendons in a vertical orientation prior to and during TLP installation. After the TLP is locked-off to the tendons and de-ballasted to tension the tendons, the TSBs are usually neither required nor desired to be carried on the tendons, as they increase wave loading on the TLP.
Tendons are subject to a number of competing design criteria, including considerations such as TLP size and design, expected environmental loads from wind and currents, the amount of allowed set-down and depth of water at the mooring location, the number of mooring tendons, tendon material, corrosion effects, and cost concerns. Aside from these considerations, tendon design usually strikes a compromise between the cross-sectional area of the pipe required for tensile strength (which can be expressed in terms of outer diameter and wall thickness), the wall thickness required to withstand bending moments, the ability to withstand external crushing force of the sea pressure at depth (which is a function of the outer diameter to wall thickness ratio, D/t), and buoyancy (which also can be expressed in terms of D/t, where greater D/t results in greater buoyancy and D/t equal to about 30 indicates a neutrally buoyant steel tendon). D/t ratios may thus vary along the length of the tendon (12) to achieve the desired overall tendon characteristics.
As deep water production progresses and the mooring depth increases, the lower portions of the tendons are subjected to increased hydrostatic pressure, which can cause buckling or crushing of the tendon. To prevent tendon failure under high seawater pressures, a strength of materials analysis shows that smaller D/t ratios are required for those portions of the tendon (12) located in deep water. In other words, without changing the tendon, material, tendons require smaller outer diameters and/or greater wall thicknesses. Unfortunately, greater wall thicknesses, coupled with longer length of the tendons, can result in tendon weights that exceed a TLP's support capacity. Larger TLPs may be required to support the heavier tendons, but larger TLPs may not be cost effective.
Alternatively, instead of increasing wall thickness, the lower portions of the tendons can be pressurized to balance the net tendon pressure at depth, as taught by U.S. Pat. No. 4,521,135 issued to Silcox and U.S. Pat. No. 6,682,266 issued to Karal et al., or the interior of the lower portion of the tendons can flooded with seawater in free communication with the exterior environment, as taught by U.S. patent application Ser. No. 4,630,970 issued to Gunderson et al. and U.S. Pat. No. 5,683,206 issued to Copple. Composite and fiber tendons are also known in art that attempt to address the problem associated with heavier tendons as the depth increases. See, for example, U.S. Patent Publication No. 2005/0244231 for Liao et al.
A preferred method for reducing tendon weight is to increase the buoyancy of the tendon, usually by increasing the volume of displaced water. This is often accomplished by strapping permanent buoyancy modules near the tops of the tendons. Alternatively, buoyancy of the tendon may be increased by increasing the tendon outer diameter, usually along the upper portion of the tendon, and in particular, at the of the tops of the tendons. However, it is usually not desirable to increase the diameter of the tendon near the sea floor, where the tendon is subjected to increased hydrostatic pressure. Thus, hollow, sealed, and stepped-diameter tendons, having sea level atmospheric pressure air therein, are known in the art. These stepped tendons seek to achieve neutral or slightly positive tendon buoyancy, by having an uppermost section with a large outer diameter and positive buoyancy that compensates for the negatively buoyant lower section that has a smaller diameter and thicker wall.
For example, U.S. Pat. No. 6,851,894 issued to Perret et al. discloses a tendon having an upper section of large diameter, an intermediate section of smaller diameter, and a lower section of smallest diameter. The upper section attaches to the TLP. Due to the large diameter of the upper section, that section is positively buoyant. The buoyancy of the upper section compensates for the weight of the heavy lower section so that the overall buoyancy of the tendon is close to neutral.
A drawback of having larger diameter tendons at the TLP is that the TSBs used during installation must in turn be enlarged to fit around the upper tendon section, and the larger tendons have increased surface area subjected to waves and current. Furthermore, if vortex fairings are to be used, they must also be larger.
3. Identification of Features Provided by Some Embodiments of the Invention
A primary object of the invention is to provide a tendon with an uppermost section that has a reduced diameter that allows smaller tendon support buoys and/or vortex fairings to be used, resulting in lower cost.
Another object of the invention is to provide a tendon with an uppermost section that has a reduced diameter that is results in reduced drag due to waves and currents.
Another object of the invention is to provide a tendon with reduced weight in water to reduce loading on the TLP.
Another object of the invention is to provide a stepped tendon, with the benefits thereof, but that retains advantages of having a reduced diameter uppermost section.